Reminds me of a story about two amateurs.
One sais:
- Hey man I bought a peace of land next to my property to set a 500 meter beverage.
- Oh! That must have cost you a fortune!
- Not at all! It is 500 meters long and 10 centimeters wide!

Well to the question:
Have a look at the diagram. L is only one leg of four. So the length of the antenna is going to be a bit shorter then double L. And as you can see the diagram shows figures for 10 Mhz.
So if I were you I’d start thinking about either vertical or switchable 2 el Delta loop like this:http://dl2kq.de/mmana/4-3-37.htm - very last one remeasured to 80 meters.

The late ZL2BE had 3-wire rhombic antennas (3 wires on each leg instead of the more common single wire) that were 20 wavelengths on a side at the lowest frequency. Since rhombic antennas operate on "powers of 2" in terms of frequency he had to have separate rhombics for 15 meters. His "main" rhombics were for 160, 80, 40, 20, and 10 meters (1, 2, 4, 8, and 16 times the frequency). Since the 15 meter band is 12 times the 160 meter frequency an antenna "cut" for 160 meters would not work.

These rhombics were installed on wooden poles that were 90 feet above ground. Needless to say, he had an excellent signal. Back in the "AM Days" the 15 meter band would appear to be completely "dead". However, as you tuned across the band your "S" meter would suddenly be "pinned" and there was ZL2BE. No other stations could be copied but he was definitely "armchair copy".

Now he was one of the largest, if not "the" largest, sheep ranchers in New Zealand and could definitely afford the very extensive antenna system that he had.

The late ZL2BE had 3-wire rhombic antennas (3 wires on each leg instead of the more common single wire)

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Every rhombic antenna I ever worked on professionally was a 3 wire design. The 1 wire design is a amateur "get by", for when you don't have the $$ to do it right.

that were 20 wavelengths on a side at the lowest frequency. Since rhombic antennas operate on "powers of 2" in terms of frequency he had to have separate rhombics for 15 meters. His "main" rhombics were for 160, 80, 40, 20, and 10 meters (1, 2, 4, 8, and 16 times the frequency). Since the 15 meter band is 12 times the 160 meter frequency an antenna "cut" for 160 meters would not work.

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Powers of 2? HUH? often the pros will "nest" 2 rhombics inside one another, you can get 2 antennas on the same patch of ground, obviously, the larger one will have a greater gain especially at lower frequency's. It's simply a way to get 2 antennas for the (land) price of 1.

These rhombics were installed on wooden poles that were 90 feet above ground. Needless to say, he had an excellent signal. Back in the "AM Days" the 15 meter band would appear to be completely "dead". However, as you tuned across the band your "S" meter would suddenly be "pinned" and there was ZL2BE. No other stations could be copied but he was definitely "armchair copy".

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That's not how it works at all.

Double the size of a rhombic and you get 3 DB more gain.

A VERY LARGE rhombic antenna might give you 20DBd of gain in the upper HF bands, about 3-4 s-units over a dipole. figuring about 60-80 db range between "dead" and "pinned", You would need a rhombic about the size of New Zealand to do this mythical signal.

Now he was one of the largest, if not "the" largest, sheep ranchers in New Zealand and could definitely afford the very extensive antenna system that he had.

Glen, K9STH

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Rhombics CAN produce significant gain. They take HUGH amounts of real estate to do it, dozens, or even the better part of hundreds of acres depending on the frequency.

They also cover a enormous frequency range, and are VERY useful to "ride the MUF" (Maximum usable frequency) for point-to-point service.

I maintained 8 dual nested rhombic antenna arrays on Diego Garcia. (4 rx,4 tx)They were pointed at Alice Springs Australia, and Asmara Ethiopia. They had a peak gain of just about 20DBd in the upper HF band.

The antennas were over 1000 feet long. And about 100' up

Because of the nulls in the radiated beam, they were POOR antennas for communicating with the fleet underway, as a few hundred nautical miles difference in the ships position on the 1st hop would easily put you from the +20DBd lobe into the -20 DBd null BETWEEN the lobes.

They were about 2 s-units louder, on average, than a 3 el 20M Yagi @ 50 ft. Hardly a signal that would pin the s-meter when nothing else was getting thru.

Actually, when I was hearing/working ZL2BE I didn't even have an "S" meter! Like a good number of amateur radio operators during the late 1950s and into the 1960s we used our "ears" to determine signal strength. Frankly, my Hallicrafters S-85 didn't have an "S" meter although its slightly bigger brother, the SX-99, did have one.

However, the signal from ZL2BE often was the only signal heard on the 15 meter band, especially during the evening hours CST, and he was always strong. I understand that he had a very strong signal on 20 meters as well. But, since my antenna, at the time, was a "true" Zepp cut for 160 meters and I didn't have a yagi on 20 meters, I generally didn't work much phone on the band. However, my 15 meter 2-element yagi did a pretty good job. By the way, my transmitter was a Heath DX-100.